1. Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting display.
2. Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (“OLEDs”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The organic light emitting display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels depending on a gray scale of video data. Each pixel includes a driving thin film transistor (TFT) for controlling a driving current flowing in the OLED. There occurs a deviation in electrical characteristics (including a threshold voltage, a mobility factor, etc.) of the driving TFT of each pixel because of a process deviation, etc. of the organic light emitting display. Hence, the pixels have different currents (i.e., different emission amounts of the OLED) with respect to the same data voltage. As a result, the organic light emitting display has a luminance deviation.
To solve the luminance deviation, an external compensation method is known to sense changes in a characteristic parameter (for example, a threshold voltage and a mobility) of the driving TFT of each pixel and to properly correct input data depending on the sensing result. The external compensation method reduces the luminance non-uniformity resulting from changes in the electrical characteristic of the driving TFT.
The electrical characteristic of the driving TFT continuously changes during a drive of the driving TFT. Thus, it is preferable to compensate for the changes in the electrical characteristic of the driving TFT in real time for an increase in a compensation performance. FIG. 1 shows a related art RT (real-time) compensation technology compensating for changes in the electrical characteristic of the driving TFT in real time using the external compensation method. As shown in FIG. 1, the related art RT compensation technology performs a sensing operation in a vertical blank period VB excluding an image display period DP from an image frame. Namely, the related art RT compensation technology senses only one display line in the vertical blank period VB of each image frame. First pixels of a display line, on which the RT sensing is not performed, maintain an emission state resulting from image display data during one image frame including the vertical blank period VB. However, second pixels of a display line, on which the RT sensing is performed, stop the emission resulting from the image display data in the vertical blank period VB, so as to perform the sensing operation. When the sensing operation is completed, luminance recovery data of the same voltage level as the image display data is input to the second pixels. The second pixels maintain an emission state resulting from the luminance recovery data during a remaining period excluding the vertical blank period VB from the one image frame.
In pixels of the display line, on which the RT sensing is performed, an emission duty resulting from the image display data in one image frame has a maximum value in one side (for example, an upper part of a display panel in FIG. 1) of the display panel, to which data is firstly applied, and gradually decreases as the display line goes from the one side of the display panel to the other side (for example, a lower part of the display panel in FIG. 1) of the display panel, to which the data is last applied. On the contrary, in the pixels of the display line, on which the RT sensing is performed, an emission duty resulting from the luminance recovery data in one image frame has a minimum value in one side (for example, the upper part of the display panel in FIG. 1) of the display panel and gradually increases as the display line goes from the one side of the display panel to the other side (for example, the lower part of the display panel in FIG. 1) of the display panel.
However, even when the image display data and the luminance recovery data are applied at the same voltage level, luminances of the image display data and the luminance recovery data represented for the same period of time are different from each other. A reason to generate such a luminance deviation is because gate signals for applying the image display data and the luminance recovery data to the pixel are different from each other. Further, the reason is because an initialization state of a source node of the driving TFT for programming the image display data is different from an initialization state of the source node of the driving TFT for programming the luminance recovery data.
As described above, when the luminance represented by the image display data is different from the luminance represented by the luminance recovery data, there occurs a luminance deviation between a display line, on which the RT sensing is performed, and display lines, on which the RT sensing is not performed, during the same image frame. Namely, as shown in FIG. 2, a luminance of one display line, on which the RT sensing is performed, may be greater or less than a luminance of one display line, on which the RT sensing is not performed.
The luminance deviation varies depending on a display location of the display line, on which the RT sensing is performed. When the display line, on which the RT sensing is performed, is positioned at the upper part of the display panel, a length of an emission period of the luminance recovery data is short. Hence, the luminance deviation is relatively small. However, as the display line, on which the RT sensing is performed, approaches the lower part of the display panel, the length of the emission period of the luminance recovery data increases. Hence, the luminance deviation gradually increases.